Mass spectrometry is a powerful method for identifying analytes in a sample. Applications are legion and include identifying biomolecules, such as carbohydrates, nucleic acids and steroids, sequencing biopolymers such as proteins and saccharides, determining how drugs are used by the body, performing forensic analyses, analyzing environmental pollutants, and determining the age and origins of specimens in geochemistry and archaeology.
In mass spectrometry, a portion of a sample is transformed into gas phase analyte ions. The analyte ions are typically separated in the mass spectrometer according to their mass-to-charge (m/z) ratios and then collected by a detector. The detection system can then process this recorded information to produce a mass spectrum that can be used for identification and quantitation of the analyte.
Time-of-flight (TOF) mass spectrometers exploit the fact that in an electric field produced in the mass spectrometer, ions acquire different velocities according to the their mass-to-charge ratio. Lighter ions arrive at the detector before higher mass ions. A time-to-digital converter or a transient recorder is used to record the ion flux. By determining the time-of-flight of an ion across a propagation path, the mass of ion can be determined.
Several methods exist for introducing the ions into the mass spectrometer. For example, electrospray ionization (ESI) offers a continuous source of ions for mass analysis. Another ionization method producing a quasi-continuous source of ions is matrix-assisted laser desorption/ionization (MALDI) with collisional cooling, sometimes referred to as “orthogonal MALDI”. In orthogonal MALDI, an analyte is embedded in a solid matrix, which is then irradiated with a laser to produce plumes of analyte ions, which are cooled in collisions with neutral gas and may then be detected and analyzed.
In ESI and orthogonal MALDI TOF systems, a portion of a sample is ionized to produce a directional source beam of ions. To couple a continuous ion source to the inherently pulsed TOF mass analyzer, the orthogonal injection method is used as described, for example in (Guilhaus et al., Mass Spectrom. Rev. 19, 65–107 (2000)). A sequence of electrostatic pulses act on the source beam to produce a beam of packets of analyte ions that are then detected and analyzed according to time-of-flight methods known to those of ordinary skill. The pulses exert a force on the ions that is generally orthogonal to the direction of the source beam and that launches packets of ions towards the detector.
The timing of the pulses is important. A waiting time must elapse between pulses to ensure that the packets of ions do not interfere with each other. Thus, there is a sequence of pulsing and waiting, which continues until a sufficient number of packets are launched from the sample. The detector detects the packets and a time-of-flight analysis can be performed to discern the composition of the sample.
The waiting time between pulses must be long enough to ensure that the packets do not interfere with each other at the detection site. In particular, the waiting time must be long enough to ensure that the lighter and faster ions of a trailing packet will not pass the heavier and slower ions of a preceding packet, which would result in some overlap of the packets. For this reason, in the traditional pulse-and-wait approach, the release of an ion packet is timed to ensure that the heaviest ions of a preceding packet reach the detector before any overlap or “crosstalk” can occur, which overlap could lead to spurious mass spectra. Thus, the periods between packets are relatively long.
Aside from resulting in a longer analysis time, long waiting times between pulses also result in sample waste. In particular, in ESI and orthogonal MALDI, the production of ions is (quasi) continuous. Thus, between pulses, the production of ions by these two methods is essentially incessant. The ions that are not pulsed during the waiting time are not detected because they do not reach the detector. Consequently, the ions that are not pulsed are wasted. When the sample being tested is in short supply or is expensive, waste of the sample material can present a serious problem.